Question 17.5: Preparing a Buffer Which acid would you choose to combine wi...
Preparing a Buffer
Which acid would you choose to combine with its sodium salt to make a solution buffered at pH 4.25? For the best choice, calculate the ratio of the conjugate base to the acid required to attain the desired pH.
chlorous acid (HClO_2) pK_a = 1.95 formic acid (HCHO_2) pK_a = 3.74
nitrous acid (HNO_2) pK_a = 3.34 hypochlorous acid (HClO) pK_a = 7.54
The best choice is formic acid because its pKa lies closest to the desired pH. You can calculate the required ratio of conjugate base (CHO_2^-) to acid (HCHO_2) by using the Henderson–Hasselbalch equation as follows:
pH = pK_a +\log\frac{[base]}{[acid]}
4.25 = 3.74 +\log\frac{[base]}{[acid]}
\log\frac{[base]}{[acid]}=4.25 – 3.74
= 0.51
\frac{[base]}{[acid]}=10^{0.51}
= 3.24